The present invention relates to a screw compressor, and more particular, to a large capacity screw compressor that generates a compressed air.
Screw compressors comprise a male rotor and a female rotor, of which rotating shafts are in parallel to each other and which rotate so that spiral teeth thereof mesh with each other, and a casing that accommodates therein the male rotor and the female rotor. A plurality of compressive working chambers are defined by tooth grooves of the male rotor and the female rotor, and an inner wall of the casing. The compressive working chambers are decreased in volume to compress an air while moving in an axial direction as the male rotor and the female rotor rotate.
Conventionally, the construction for a two stage type screw compressor is exemplarily disclosed, which comprises: a low pressure stage compressor body; an intercooler that cools a compressed air from the low pressure stage compressor body; a high pressure stage compressor body that further compresses the compressed air cooled by the intercooler; and an aftercooler that cools the compressed air from the high pressure stage compressor body (for example, see JP-A-2002-155879). According to the related art, pinion gears, respectively, are mounted on rotor shafts (either of a male rotor and a female rotor) of the low pressure stage compressor body and the high pressure stage compressor body. The pinion gears, respectively, mesh with a bull gear, which is mounted on a rotating shaft of a motor (electric motor). As the motor is driven, a rotational power of the motor is transmitted and increased in speed through the bull gear and the pinion gears, whereby the low pressure stage compressor body and the high pressure stage compressor body, respectively, are driven.
However, the related art leaves the following room for improvement.
That is, according to the related art, a speed increasing ratio is determined by a ratio of a working pitch diameter of the bull gear on a side of the motor to a working pitch diameter of the pinion gear on a side of the compressor body, and a rotational power of the motor is increased in speed in one stage according to the speed increasing ratio to drive the low pressure stage compressor body and the high pressure stage compressor body, respectively. Therefore, in order to obtain a predetermined speed increasing ratio in a compressor unit of a large capacity with, for example, an output of several hundreds of kilowatts (kw), it is necessary to increase a diameter of the bull gear on the side of the motor corresponding to the pinion gear on the side of the compressor body, or to decrease a speed increasing ratio to heighten the rotational speed of the motor. In the case where the gear is to be made large in diameter, manufacture becomes sometimes difficult in terms of a manufacturing facility (for example, limitation in a range of working performed by a machine tool). Consequently, the gears or the motor is increased in cost.